1. Field of the Present Invention
The present invention relates to an organic electroluminescence device (hereinafter, referred to as an “organic EL device” in some cases) which can be effectively applied to a surface light source for full color display, backlight, illumination light sources and the like; or a light source array for printers, and the like.
2. Description of the Related Art
An organic EL device is composed of a light-emitting layer or a plurality of organic layers containing a light-emitting layer, and a pair of electrodes sandwiching the organic layers. The organic EL device is a device for obtaining luminescence by utilizing at least either one of luminescence from excitons each of which is obtained by recombining an electron injected from a cathode with a positive hole injected from an anode to produce exciton, or luminescence from exciton of other molecules produced by energy transmission from the above-described excitons.
Heretofore, an organic EL device has been developed by using a laminate structure from integrated layers in which each layer is functionally differentiated, whereby the brightness and the device efficiency are remarkably improved. For example, it is described in “Science”, vol. 267, No. 3, page 1332; 1995 that a two-layer laminated type device obtained by laminating a positive hole-transport layer and a light-emitting layer also functioning as an electron-transport layer; a three-layer laminated type device obtained by laminating a positive hole-transport layer, a light-emitting layer, and an electron-transport layer; and a four-layer laminated type device obtained by laminating a positive hole-transport layer, a light-emitting layer, a positive hole-blocking layer, and an electron-transport layer have been frequently used.
For the practical application of an organic EL device, however, there are still many problems such as improvements in luminous efficiency, and driving durability. Particularly, increase in luminous efficiency results in a decrease in electricity consumption, and further, it is advantageous in view of driving durability. Accordingly, many means of improvement have been heretofore disclosed. However, a luminescent material having a high luminous efficiency usually has a disadvantage of causing brightness deterioration during driving thereof, and further, a material excellent in driving durability involves a disadvantage of low brightness. Accordingly, it is not easy to achieve both higher in luminous efficiency and higher driving durability, and thus, further improvements are sought. For instance, it is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2000-315581 that an inorganic electric charge barrier layer is arranged between either a light-emitting layer and both electrodes, or the light-emitting layer and one of the electrodes, whereby carrier injection efficiency is increased. In Japanese Patent Application National Publication No. 2002-532848, it is also disclosed that a barrier layer containing an insulating material is disposed on an electrode material or adjacent to the electrode. In JP-A No. 9-148071, it is disclosed that an intermediate layer prepared by dispersing an inorganic fine powder in a polymer binder is arranged. However, these layers to be newly arranged, starting the inorganic electric charge barrier layer, disturb charge transfer by means of carriers, and thus, it has not been possible to use them in the form of an ultra-thin layer of around several nm.
In JP-A No. 2002-56988, configuring a diffraction grating prepared by disposing a high refractive index medium layer between an anode and a light-emitting layer is proposed. However, the high refractive index medium layer also inhibist charge transfer by means of carriers, so that it has only been possible the high refractive index medium layer in the form of an ultra-thin layer.
On the other hand, a means for improving extraction efficiency of the light emitted in a light-emitting layer is also disclosed. For instance, in Japanese Patent Application Laid-Open No. 2003-257620, there is disclosed a light loss prevention layer having a plurality of regions each of which has a refractive index different from one another between the light-emitting layer and the electrode. The light loss prevention layer contains, for example, a mixture composed of a high refractive index material and a low refractive index material wherein their refractive indexes differ from one another by 0.3 or more, whereby a sea-island structure composed of high refractive index phases and low refractive index phases in the layer is present, and it is attempted to improve light extraction efficiency by means of a light scattering effect. However, there is a disadvantage in that such a light loss prevention layer disturbs charge transfer of electric charge in the device, resulting in a decrease in quantum efficiency of the luminescence, whereby the driving voltage increases.